Organizing centers in the Developing brain
The metencephalon develops into the cerebellum and the pons whereas the mesencephalon develops into the midbrain. The border between them is a group of specialized cells known as the isthmus organizer. It releases several signaling molecules such as engrailed which is a homeodomain transcription factor. This secreted molecule, along with other factors such as Engrailed-1 and Engrailed-2 are responsible for moulding and patterning the neuroepithelium.
Illustration 1 modified from Gilbert 6 ed., Fig 12.10
The isthmus organizer is set up due to the expression of Otx2 in the prosencephalon and mesencephalon – its posterior limit of expression marks the anterior limit of midbrain-hindbrain boundary (MHB) - and Gbx2 which is expressed in the rostral part of the hindbrain – its anterior limit of expression marks the posterior limit of MHB. A member of the Hox gene family (Hoxa2) suppresses cerebellar development in the caudal hindbrain and all of these factors together give rise to the mature cerebellum.
The metencephalon can pattern neural tube to adopt more posterior activity. (Alvardo-Mallart et al., 1984)
Signaling molecules such as Wnt1 (KO mice, McMohan & Bradley, 1990), Fgf8 and Engrailed act together in an interconnected network such that deleting one of them affects the others. Fgf8 is a fibroblast growth factor and acts as an organizer as it sets up the MHB and is crucial in maintaining it (Crossley et al. 1996).
Paired box genes such as Pax2 and Pax5 are also required for differentiation and patterning of the midbrain and hindbrain. Mutations in these genes cause deletions of mesencephalic and cerebellar structures.
Forebrain development, prosomeres and Pax genes
The prosencephalon later develops into the forebrain. It is subdivided into 6 prosomeres where P1 is rostral (adjacent to mesencephalon) and P6 is caudal.
P2,3 differentiate into the diencephalon and P4,5,6 form the telencephalon. The telencephalon is the source of progenitors that give rise to the cerebral cortex.
Pax genes encode transcription factors involved in specifying regional identities. In the cortex, specifically Pax6 and also emx2 are responsible for this. They are expressed in opposing gradients with emx2 being in the caudo-medial pole, responsible for the development of the visual cortex and pax6, expressed in the rostral-lateral pole, responsible for the development of the frontal and motor cortex.
Fgf8 is expressed in the anterior pole. Increasing it causes the downregulation of emx2 and a caudal-ward shift of cortical regions, whereas decreasing Fgf8 causes a rostral-ward shift in the cortical regions.
The generation of the cerebral cortex proceeds in a medial to lateral fashion where neurons are arranged in different lamina.
(More about how these distinct layers are formed in the migration section.)
Figure 1 modified from "Development of the Nervous System" 2nd Ed, Fig. 3.1
The neural tube is initially one cell thick. Then progenitor cells undergo lots of divisions to produce several layers.
The neural tube is divided into 3 zones:
- ependymal (known as "ventricular") zone which is the innermost, near the ventricle,
- mantle zone (intermediate) and,
- marginal zone which has post-mitotic neurons undergoing nuclear translocation.
The cells in these zones are in different stages of the cell cycle. Growth factors generally act (as mitogens) to control the progression from G1 to S phase of the cell cycle. Increased dependence on growth factors lengthens the G1 phase and hence the overall cell cycle.
The number of cells formed by the precursor cell in the ventricular zone depends on two things:
- The stage of development and,
- The region of the nervous system where the progenitor is located.
In the early embryonic cerebral cortex the number of cells more than doubles each day. This stage of symmetric division is known as the “expansion phase”, where the progenitor cells act like stem cells.
As the cell-cycle time gradually becomes longer, fewer new cells are generated each day. In the later stages, a greater proportion of the progenitor cells differentiate into neurons and glia. The sequences of generation of different types of neurons and glia are well conserved and orderly as shown in Fig 2, with origination occurring in an invariant timetable. Large neurons are generated before small neurons in the same region (eg pyramidal cells become post-mitotic before granule cells).
Figure 2 modified from "Development of the Nervous System" 2nd Ed, Fig. 3.4
Cell-cycle genes control the number of neurons generated during development
Complicated sequences of protein interactions control and co-ordinate the progress of a cell through the stages of cell replication. This is conserved amongst species. Dramatic changes in the expression levels of cyclins correlate with specific stages of the cell cycle. Cyclin-dependant kinases (CDKs) together with cyclins are necessary for the progression of a cell to the next stage in the cell cycle.
Cell interactions control the number of cells made by the progenitors
FGFs, TGF-alpha, EGF and insulin-like growth factors all influence progenitor cells. Other signaling molecules such as Shh and Wnt are involved in the regulation of progenitor proliferation at later stages of brain development. They use extensive feedback mechanisms to maintain the proliferation of progenitor cells and ensure that the correct number of neurons are made during development.
Figure 3 modified from "Development of the Nervous System" 2nd Ed, Fig. 3.10
The generation of neurons and glia
Complex interactions of extracellular signaling factors might also direct progenitor cells to either a neuronal or glial lineage. The Notch signalling pathway is responsible for lateral inhibition of cell fates during early stages of brain development. If the pathway is active, astrocytes are formed (also refer to Fig. 2).
Neural progenitor cells express several proneural genes including neuroD1, Neurogenin, and Mash1. These, in part, determine whether the progenitor cell makes a neuron or a glial cell. Overexpression of these genes forces the cell to adopt a neuronal cell fate.
Figure 4 modified from "Development of the Nervous System" 2nd Ed, Fig. 3.1